Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS) and a plurality of subscriber units. An established harmonised cellular telephony communication system, providing predominantly speech and short-data communication, is the Global System for Mobile Communications (GSM). GSM is often referred to as 2nd generation cellular technology.
An enhancement to this cellular technology has been developed, termed the General Packet Radio System (GPRS). GPRS provides packet switched technology on GSM's switched-circuit cellular platform. A yet further enhancement to GSM that has been developed to improve system capacity can be found in the recently standardised Enhanced Data Rate for Global Evolution (EDGE) that encompasses Enhanced GPRS (EGPRS). A still yet further harmonised wireless communication system currently being defined is the universal mobile telecommunication system (UMTS). UMTS is intended to provide a harmonised standard under which cellular radio communication networks and systems will provide enhanced levels of interfacing and compatibility with many other types of communication systems and networks, including fixed communication systems such as the Internet. Due to this increased complexity, as well as the features and services that it supports; UMTS is often referred to as a third generation (3G) cellular communication technology. In UMTS subscriber units are often referred to as user equipment (UE).
Within GSM, two modes of operation (i.e. modulation schemes) may be used, Gaussian Minimum Shift-keyed (GMSK) modulation and 8-phase shift keyed (8-PSK) modulation. GMSK is a constant amplitude phase modulation scheme whilst, for the second generation (2G) standard, 8-PSK incorporates both amplitude and phase modulation.
One feature associated with most present day wireless communication systems allows the transceivers in either or both the base station and/or subscriber unit to adjust their transmission output power to take into account the geographical distance between them. The closer the subscriber unit is to the base transceiver station's (BTS's) transceiver, the less power the subscriber unit and BTS's transceiver are required to transmit, for the transmitted signal to be adequately received and decoded by the other unit. Thus, the transmit power is typically controlled, i.e. set to a level that enables the received signal to be adequately decoded, yet reduced to minimize potential radio frequency (RF) interference. This ‘power control’ feature saves battery power in the subscriber unit. Initial power settings for the subscriber unit, along with other control information, are set by the information provided on a beacon (control) physical channel for a particular cell.
Furthermore, in a number of wireless communication systems, the effect of fast fading in the communication channel is a known and generally undesirable phenomenon caused by a desired signal arriving at a receiver via a number of different paths. Therefore, fast power control loops are often adopted to rapidly determine and optimize the respective transmit power level.
It is known that within the field of power control techniques, particularly in Gaussian minimum shift keyed (GMSK) systems that employ inner power control loops, a power amplifier (PA) that ramps down from a saturated condition is likely to compromise its switching output radio frequency spectral (ORFS) performance. Here, it is worth clarifying that when the PA is in a saturated condition, the PA output power does not change in response to any change in either the PA control voltage point or input radio frequency, RF, signal level. Thus, the so called ‘control slope’ (sometimes referred to as ‘control gain’), which equates to a ratio of a change in the PA output power level to an input power control to the PA (which in this specific embodiment is the rate of change of the detected power (output of detector logic 330) to the input bias (output of 426), is essentially zero.
As the control slope tends towards zero, for example as the PA saturates, the closed loop gain, in turn, also tends towards zero. As the closed loop gain tends towards zero, the transmitter loop bandwidth collapses and the loop latency (response time) increases. Notably, with zero bandwidth, the transmitter system is unable to ramp down in a controlled manner. Additionally, in practice as the loop is commanded to ramp down at the end of a slot, there is initially no response from the loop and followed by a sudden rapid turn off.
As known in the art, such an uncontrolled ramp down results in a high frequency content in the PA's spectral response is likely to satisfy switching output (power) radio frequency spectral (performance) (ORFS) specifications. Thus, it is necessary to provide a saturation detection scheme to ensure that the output power is backed off to an unsaturated PA region before rampdown commences. Known saturation detection and correction techniques either fail to work well in practice or the proposed implementation adds extra requirements to the PA, thereby making the solution very specific to a given family of PAs or specific PA vendors.
It is known that two distinct types of specific saturations exist within GMSK systems (that have inner loops). The first type of saturation is known as hard saturation, where the PA is unable to deliver the target output power, for example due to the control slope reducing to zero. The second type of saturation is known as soft saturation, where the PA can deliver the required target power, but the response of the system is so sluggish that it causes ramp down switching ORFS problems. Here, the control slope ‘tends’ towards, but is not yet, zero. In soft saturation, the control slope is low enough that the loop bandwidth is still below a critical threshold value that means the switching ORFS specification is compromised. Therefore, it is a general requirement within communication units to provide two types of saturation detection, namely hard saturation detection and soft saturation detection.